1. Field of the Invention
The present invention relates to an implantable heart stimulating device such as a pacemaker, defibrillator or cardioverter, having electrode leads having stimulation surfaces adapted to apply stimulation energy to heart tissue.
2. Description of the Prior Art
An early indication of lead insulation defects, broken/damaged lead conductors and loose set screws/improper lead connection may be provided by the intermittent occurrence of so-called make-break signals, which then may remain unrecognized for a longer period of time. Even extensive follow-up evaluation of a sporadic symptomatic patient often fails to disclose indications of an imminent lead or lead connector failure, since the make-break signals might only be prevailing in a certain, specific body position.
Make-break signals may arise when:                The lead conductors make intermittent contact with each other due to an inner lead insulation defect/damage.        The outer conductor makes intermittent contact with the pulse generator housing due to an outer insulation defect/damage (caused by the lead insulation being rubbed off against the pulse generator housing).        The outer conductor makes intermittent contact with the outer conductor of another lead due to the fact that the leads have crossed each others in the pulse/ICD pocket and rubbed off each others insulations.        The two ends of a broken lead conductor meet intermittently.        A loose set screw.        A lead connector pin not fully inserted into the pulse generator connector.        The electrode(s) or outer insulation defects touches the electrodes of another lead inside the heart.        
Make-break signals may cause:                Inhibition/triggering of pacemaker stimulation.        Noise mode reversion with risk for T-wave stimulation.        Triggering of inappropriate ICD therapy.        
It is realistic to assume, that lead insulation/conductor damages as well as connector problems in pacemaker/ICD systems will continue to occur to some extent also in the future. According to clinical experience incipient lead and/or connector failures are often difficult to disclose at an early stage.
Thus, the development of life threatening conditions in some patients may remain unrecognized for substantial periods of time despite normal follow-up intervals. However, early disclosure of lead and connection failures is of outmost importance for patient safety and would benefit from new, preferably automatic, diagnostic approaches.
U.S. Pat. No. 5,558,098 relates to a method and apparatus for detecting lead sensing artefacts in cardiac electrograms, the sensing artefacts are caused by lead conductor fracture, lead insulation failure or connector port fluid penetration. The system includes at least two pairs of sensing electrodes which provide two distinct electrogram signals to the sensing and analysis circuitry of a pulse generator. Each signal is analyzed for heart rate. The rates are compared and if the rates detected are significantly different therapy is not delivered to the patient. In an alternative embodiment, the two signals are compared by performing a correlation analysis.
In this known device a comparison between two distinct electrogram signals from at least two pairs of sensing electrodes is performed in order to obtain an indication of e.g. a lead conductor fracture.
A drawback of this known device is that extra hardware is required, e.g. in the form of extra sensing electrodes and means for connecting the electrodes to the measurement means, in order to perform the measurement.